High performance closed loop position controllers require a high quality rate signal for stabilization and rate scheduling. DC tachometers are used almost exclusively to generate this signal. On a multi-axes robot, making provision for a DC tachomter on each axis imposes severe constraints on the mechanical design. On the hand and wrist axes of a robot, for example, the size and cost of the DC tachometer may be comparable to the drive motor, and the additional leads required by the tachometer impose design penalties. In particular with robots there is a strong incentive to minimize the number of sensors carried on the machine.
The incremental position encoder commonly used in robot and machine tool closed loop control systems generates N sinusoidal cycles per rotation of the encoder shaft. Two channels with a relative phase shift of 90.degree. are normally used to determine direction of rotation. These signals are converted to pulsed outputs and accumulated in up-down counters to yield position information.
Digital encoders have been extensively used as tachometers in velocity control systems. In these systems the encoder can be scaled to yield a high sampling rate at the nominal control velocity. Direction sense is not required, consequently counting the number of pulses per unit of time and converting to an analog voltage yields an adequate velocity signal. However, in a position control system velocity must be controlled over a wide range including through and at zero and for both negative and positive velocities. Because of the quantized output of the digital encoder a high performance system will have a limit cycle instability at or near zero velocity. The result is excessive power dissipation in power drives and rough velocity control at low velocities.
A functional block diagram of the disclosed system is similar to the function required to produce an amplitude modulated suppressed carrier radio wave. The present application and specific implementation are not suggested by the prior art.